1879. ] Obsidian in the Yellowstone National Park. ` 247 
NOTES ON AN EXTENSIVE DEPOSIT OF OBSIDIAN 
IN THE YELLOWSTONE NATIONAL PARK. 
BY WM.: H. HOLMES, 
ee ee i deposits of obsidian and obsidian porphy- 
ries had been observed in the national park previous to our 
visit in the summer of 1878, but no satisfactory exposures of the 
glassy varieties had been found. In October I had occasion to 
make examinations of a locality particularly rich in them, situated 
in the north-western part of the park, near the head of Obsidian 
or Alum creek, a tributary of the middle fork of Gardiner’s river. 
The crumbling trachytes of this part of the park give, in general, 
a rounded and monotonous character to the topography. The 
slopes of the valleys are gentle excepting at points where the 
glassy rocks predominate. 
In ascending Obsidian creek, by way of the newly-cut wagon 
road which connects Mammoth Hot Springs with the Geyser 
Basins, we pass first through broad meadows and parked forests. 
Farther on the valley narrows up and the timber becomes 
extremely dense. At a point about twelve miles above the junc- 
tion of the creek with the main stream, there is a narrow gateway 
known: as Obsidian cañon, through which the road and creek 
pass. From the east side of the valley a low promontory extends 
forward to the creek and breaks off in an abrupt nearly vertical 
wall, in which the obsidian rocks are exposed. The road 
approaches the cafion along the west side of the valley, and 
crosses to the east side at the lower end of the cafion; in order 
to avoid the swampy ground that borders the stream it has been 
carried across the steep débris slopes of the obsidian cliffs. For 
half a mile it is paved with glassy fragments and lined by 
huge angular masses of black and banded obsidian rock. From 
the upper border of the débris slope the vertical cliffs rise to the 
height of nearly two hundred feet. The lower half is composed 
of a heavy bed of black obsidian which exhibits some very fine 
pentagonal columns, somewhat irregularly arranged and frequently 
distorted, but with perfectly cut faces that glisten in the sunlight. 
The upper portion of the wall is composed of a much more 
obscurely columnar mass of impure spherulitic obsidian, the rude 
faces of the columns being often as much as ten or twelve feet — 
across, To the right and left the columnar character becomes 
E 
